1. Field of the Invention
The invention relates to the manufacture of laminated safety glass panes comprised of a sheet of glass and a sheet of flexible plastic material which latter sheet is comprised of at least one layer.
2. Description of the Prior Art
In particular, the plastic sheet comprises a single or multilayered sheet, which may be comprised, e.g., of a layer of transparent plastic material having energy-absorbent properties. Said sheet may also be comprised of a facing layer of a plastic material which resists scratching and abrasion.
The subject glass panes must satisfy a number of requirements. In order to find use, particularly in the automobile industry, e.g., as windshields, they must have excellent optical properties in addition to excellent biomechanical properties. (Biomechanical properties are those properties related to human safety considerations, such as, for example in the automobile industry, considerations relating to passenger impact upon a glass pane as the result of an automobile accident.) One of the problems encountered in the manufacture of this type of glass pane is that of the flawless application of the sheet of plastic material to the glass sheet such that the initial optical properties of said plastic sheet are preserved.
It is particularly difficult to achieve such an application operation wherein, in the course of manufacturing curved laminated glass panes, one needs to avoid wrinkling to the plastic sheet.
A number of methods of manufacturing the subject type of glass pane are known.
Generally, according to the known methods, the procedure is as follows: The different sheets which are to make up the laminated glass pane are stacked and are combined with the aid of heat and pressure, wherewith the combining operation is generally preceded by a preliminary combining operation wherein the air trapped between the sheets is evacuated.
Fr. Pat. 2,153,037, for example, discloses a method called "contre-moule" (in English, "counter-molding"), employed particularly for manufacturing laminated safety glass panes comprised of a silicate glass sheet and a flexible plastic sheet, whereby the constituent parts of the glass pane are combined with the aid of heat, possibly also under pressure, and whereby prior to any pressing or heating a protective glass sheet is applied to the stacked plastic sheet which protective glass sheet is of a shape corresponding to the shape of the glass sheet which is a constituent of the laminated glass pane being produced, and which protective glass sheet will be removed after the thermal treatment (which treatment may also be accompanied by independent pressing). The presence of the protective glass sheet (called the "counter-mold") enables one to avoid the development of defects in planarity or, in general, the development of warping during the heat and pressure treatment.
According to a variant of this "counter-molding" method of manufacture, which variant is described in particular in Fr. Pat. 2,123,089 for the manufacture of a sheet of silicate glass covered with a flexible plastic sheet with an intermediate layer of a thermo-plastic adhesive, the protective counter-mold sheet is applied during a preliminary combination operation wherein the air trapped between the constituent parts of the glass pane being produced is evacuated. This type of preliminary operation by itself leads to sufficient adhesion between the sheets to render the product pane suitable for certain applications. However, for the product to be used as a windshield, this operation must be followed by a final combination operation. The preliminary combination facilitates the final combination.
This known method of manufacturing a laminated glass pane, employing a "counter-mold", has certain advantages but at the same time is relatively costly, since a separate counter-sheet (i.e., "counter-mold piece") must be used for each laminated pane produced; moreover, in general, and particularly in the case of curved glass panes, each counter-sheet is used only once, being discarded after it is removed from the product pane, and there is the strict requirement that the curvature of the "counter-mold" be identical to that of the glass sheet which is a constituent of the laminated glass pane product.